powerpc/fsl_msi: drop unneeded cast to non-const pointer

This cast is unneeded since *of_device_id.data became const.

[ukl: split Arnd's patch by driver and add changelog]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>